The Bishop Tuff: New Insights From Eruptive Stratigraphy
The Bishop Tuff: New Insights From Eruptive Stratigraphy
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主教凝灰岩：喷发地层学的新见解
DOI：
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发表时间：
1997
期刊：
影响因子：
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通讯作者：
W. Hildreth
中科院分区：
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作者：
C. Wilson;W. Hildreth
产自加州东部长谷破火山口的0.76 Ma主教凝灰岩，由广泛的瀑布存款和大量的部分焊接熔结凝灰岩组成。落存款（F），暴露在一个东风扇区下面和附近的熔结凝灰岩，分为9个单位（F1-F9），没有明显的时间中断，除了可能在F8和F9之间。将最大碎屑尺寸与已知或推断积累速率的其他沉积物进行比较，以估计F1-F8的积累时间约为2000年。90小时熔结凝灰岩（IG）被分成按时间和/或地理不同的材料包。早期的包裹体（Ig 1）主要向东侵位，完全是内布里尼期（与F2-F8单元同时代），缺乏斑晶辉石，并且包含很少或没有玻璃山衍生的流纹岩岩屑。早期的包裹体（Ig 2）主要在北部和东部喷发，至少部分是内层（与东部的F9瀑布单元互层），含有辉石，并具有富含玻璃山衍生流纹岩或北方破火山口边缘暴露的其他岩性的岩性部分。认识到火山口以东的Ig 1的intraplinian性质和使用的秋天存款测时产生的积累估计约。对于较早、焊接较少的子包装和约25小时。36小时后，主要是焊接分装。致密焊接块状熔结凝灰岩的平均累积速率范围高达≥1 mm/s，相当于非焊接材料的平均累积速率≥2.5 mm/s。Ig 1和北方Ig 2叶内部分层的比较表明，北方熔结凝灰岩在≥35小时内积累最厚。可识别的喷口位置从先前提出的位于破火山口中南部的初始位置（F1 - 8，Ig 1）以复杂的方式迁移;一个喷口组（东部Ig 2）向东和向北迁移到玻璃山，而另一个组（北方Ig 2）从西向东穿过北方破火山口边缘。火山口西南部Ig 1和Ig 2的通风口位置尚未确定。新的地层框架表明，主教熔结凝灰岩的大部分是intraplinian的性质，和下跌存款和熔结凝灰岩单位以前推断为连续的大部分或全部同生。因此，需要对所有现有的喷发动力学模型以及母岩浆房中微量元素、挥发物和同位素的喷发前分带的性质和原因进行根本性的重新评估。
The 0.76 Ma Bishop Tuff, from Long Valley caldera in eastern California, consists of a widespread fall deposit and voluminous partly welded ignimbrite. The fall deposit (F), exposed over an easterly sector below and adjacent to the ignimbrite, is divided into nine units (F1‐F9), with no significant time breaks, except possibly between F8 and F9. Maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for F1‐F8 as ca. 90 hrs. The ignimbrite (Ig) is divided into chronologically and/or geographically distinct packages of material. Earlier packages (Ig1) were emplaced mostly eastward, are wholly intraplinian (coeval with fall units F2‐F8), Lack phenocrystic pyroxenes, and contain few or no Glass Mountain‐derived rhyolite lithic fragments. Earlier packages (Ig2) were erupted mostly to the north and east, are at least partly intraplinian (interbedded with fall unit F9 to the east), contain pyroxenes, and have lithic fractions rich in Glass Mountain‐derived rhyolite or other lithologies exposed on the northern caldera rim. Recognition of the intraplinian nature of Ig1 east of the caldera and use of the fall deposit chronometry yields accumulation estimates of ca. 25 hrs for an earlier, less‐welded subpackage and ca. 36 hrs for a later, mostly welded subpackage. Average accumulation rates range up to ≥1 mm/s of dense‐welded massive ignimbrite, equivalent to ≥2.5 mm/s of non‐welded material. Comparisons of internal stratification in Ig1 and northern Ig2 lobes suggest the thickest northern ignimbrite accumulated in ≥35 hrs. Identifiable vent positions migrated from an initial site previously proposed in the south‐central part of the caldera (F1‐8, Ig1) in complex fashion; one vent set (for eastern Ig2) migrated east and north toward Glass Mountain, while another set (for northern Ig2) opened from west to east across the northern caldera margin. Vent locations for Ig1 and Ig2 southwest of the caldera have not been identified. The new stratigraphic framework shows that much of the Bishop ignimbrite is intraplinian in nature, and that fall deposits and ignimbrite units previously inferred to be sequential are largely or wholly coeval. Fundamental reassessment is therefore required of all existing models for the eruption dynamics and the nature and causes of pre‐eruptive zonations in trace elements, volatiles, and isotopes in the parental magma chamber.